This invention relates generally to tools for removing coke from containers such as coking drums used in oil refining, and more particularly to an enhanced remotely operated cutting mode shifting apparatus for use with a combination decoking tool.
In conventional petroleum refining operations, crude oil is processed into gasoline, diesel fuel, kerosene, lubricants or the like. It is a common practice to recover heavy residual hydrocarbon byproducts through a thermal cracking process known as delayed coking. In a delayed coker operation, heavy hydrocarbon (oil) is heated to a high temperature (for example, between 900° F. and 1000° F.) in large fired heaters known as fractionation units. The heated oil releases its hydrocarbon vapors (including, among other things, gas, naphtha and gas oils) to the base of the fractionation unit for processing into useful products. The residual is then transferred to cylindrical vessels known as coke drums. These vessels, which are typically configured to operate in pairs, are as large as 30 feet in diameter and 140 feet in height. The combined effect of temperature and retention time leaves this residual, which is known as petroleum coke (or more simply, coke), in a solidified form. This coke residue must be broken up in order to remove it from the vessel, and is preferably accomplished by using a decoking (or coke cutting) tool in conjunction with a decoking fluid, such as high pressure water.
Such a tool includes a hydraulically-operated drill bit with both drilling and cutting nozzles that are configured to deliver a jet of fluid to the solidified coke. The bit is lowered into the vessel through an opening in the top of the vessel, and is formed into a common tool housing such that the high pressure water supply can be selectively routed through either the drilling or cutting nozzles, depending on the mode of operation. Since high flow rates and pressures (for example, flows of 1000 gallons per minute (gpm) at 3000 to 4000 pounds per square inch (psi)) are typically used for such operations, it is neither practical nor desirable to open drilling and cutting nozzles at the same time. Thus, to achieve this mode shifting, diverter valves or other flow control devices are used to direct the flow to the selected nozzles as required for the decoking operation. There are two commonly used diverter valve designs, both of which are complex, require numerous components, and require a very high level of precision in their manufacture in order to function properly.
One such valve is a reciprocatable sleeve-type valve having radial ports which selectively align with corresponding ports in the valve body to direct flow to either the drilling or cutting nozzles. The other is a rotatable sleeve, again having ports for selective alignment with corresponding ports of the valve body. In a more benign environment, both designs would provide adequate diversion control and operation. However, the water used in drilling and cutting operations is typically recycled repeatedly, collecting a quantity of suspended coke fines in the process. The passage of this water between seals, sleeves and related slidably-engaged adjacent tool components contributes to localized deposit of the fines. Such deposition can result in component jamming, especially between the sliding components of a reciprocating sleeve-type valve, thereby rendering the valve and the decoking tool inoperative. A similar result may also occur whether the valve is moved by springs, pneumatic or manual means. Once jammed, the tool must be removed, disassembled, and cleaned before decoking can be resumed. In fact, the overwhelming majority of failures in conventional autoshift tools are the result of coke fines entering the autoshift chamber and causing binding in the internal mechanism.
Another difficulty associated with these earlier designs is that they accomplished their mode shifting upon application of the cutting fluid pressure. Such operation is hard on the components, as the high levels of friction in the adjacent sliding or reciprocating components in such a high pressure environment made relative movement between such components more difficult. Furthermore, in situations involving the introduction of a high pressure fluid into a previously unpressurized (or underpressurized) flowpath, the possibility of a water hammer forming is enhanced. It is additionally problematic in that the already high levels of friction associated with the increasing pressure loads only increased the friction forces arising from the presence of the fines, thereby exacerbating the jamming tendency of the shuttling components.
A relatively trouble-free, manually shiftable, combination decoking tool was developed and described in U.S. Pat. No. 5,816,505, which is commonly owned by the Assignee of the present invention, and is incorporated herein by reference. The trouble-free nature of this tool is attributable to its mode shifting valve design which includes only a rotatable diverter plate for selectively directing cutting fluid to either pilot hole drilling nozzles or full-width coke cutting nozzles, thus eliminating most of the moving parts associated with other shifting mechanisms. Moreover, because of the simple rotatable flat diverter plate acting on a complementary flat diverter valve body, it also eliminated the multiple interfaces between parts which hitherto provided the jamming and associated failure sites of earlier designs of remotely operated shifting devices. In spite of these improvements, the tool still needed to be removed from the coke drum in order to change the cutting mode. To that end, an automated remotely operating decoking tool, described in U.S. Pat. No. 6,644,567, which is likewise commonly owned by the Assignee of the present invention and is incorporated herein by reference, was developed. The tool disclosed therein extends the operation of the tool disclosed in U.S. Pat. No. 5,816,505 by mounting a shifter body to the decoking tool that can rotate the tool's diverter plate upon release of the cutting fluid pressure from the tool. It would nevertheless be advantageous to extend the operability of the automated decoking tool disclosed in U.S. Pat. No. 6,644,567.